HighlightsRotary knives were added to an ear-snapper header to increase corn stover yield in a single-pass biomass system.Stover yield increased with the number of knives but at the expense of combine productivity and fuel consumption.Bale moisture was often greater than would be considered appropriate for good aerobic conservation.Abstract.Modifications were made to a conventional ear-snapper corn header to increase corn stover yield when a single-pass round baling system was integrated with a combine harvester. To collect more leaves and top portions of stalks, knives oriented parallel to the deck plates were added to shear crop material above the ear-snapper rolls. Stover yield was primarily altered by the number of knives on the header; and to a lesser extent by the fore-and-aft position of the knives and the header height. The number of knives on a 12-row header was varied from two to six in increments of two. Stover yield increased linearly with the number of knives, and dry basis stover yield ranged from 1.1 Mg ha-1 (no knives) to 3.6 Mg ha-1 (six knives) over the five years of data collected (2012 to 2016). Combine productivity decreased linearly and specific fuel consumption increased linearly with greater stover yield. Combine productivity declined by as much as 50% when six knives were used. Dry basis bale density decreased linearly with the number of knives because the dense cobs became a smaller fraction of the total bale mass. In three of the five years, bale moisture increased linearly with the number of knives; in those three years, bale moisture was typically greater than 30% (wet basis). Adding knives to the header increased single-pass stover yield but at considerable cost to combine harvester productivity, and aerobic bale conservation would be challenged by high bale moisture. Keywords: Baling, Combine, Corn, Density, Moisture, Productivity, Stover, Yield.